1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which is configured to surely discharge static electricity charged in a front surface side of a liquid crystal display panel to a ground member such as a metal frame.
2. Related Art
The liquid crystal display device is configured such that a backlight assembly, a display control circuit board and the like are combined to a liquid crystal panel and these parts are formed into an integral body using a frame. The liquid crystal panel is configured such that a first substrate, that is, a thin film transistor substrate (TFT substrate) which forms pixel circuits each constituted of a thin film transistor on a main surface thereof and a second substrate, that is, a counter substrate (CF substrate) which forms color filters and the like on a main surface thereof and constitutes an image display surface side are adhered to each other, and liquid crystal is sealed in a gap formed between these substrates.
Here, in a vertical-electric-field-type liquid crystal display device which is also referred to as a TN-type liquid crystal display device, a common electrode (counter electrode) is formed on a main surface of a counter substrate. On the other hand, an IPS-type (lateral-electric-field-type) liquid crystal display device forms no electrodes on a main surface of a counter substrate. Further, beside these liquid crystal display devices, there has been known a liquid crystal display device which forms color filters on a TFT substrate and forms no color filters on a counter substrate, and a so-called VA type liquid crystal display device. Here, for the sake of brevity, the explanation is mainly made with respect to a liquid crystal display device in which the counter substrate is formed of the CF substrate. A display region occupies a most portion of an area of the liquid crystal panel, and a drive circuit and the like are mounted or formed on an outer peripheral portion of the display region. Further, an illumination device (backlight assembly) referred to as a backlight for making an image formed within the display region clear is usually mounted on a back surface of the liquid crystal panel.
The TFT substrate and the CF substrate which constitute the liquid crystal panel respectively include an insulation plate made of glass or the like. Particularly, the TFT substrate forms electrodes for driving pixels and the like on a main surface thereof. Further, a polarizer is adhered to an outer surface of the TFT substrate. Particularly, to an outer surface of the substrate (CF substrate) on a display screen side, static electricity is charged due to the application of a drive voltage to the liquid crystal. In the so-called IPS-type liquid crystal display device, in view of a display mode thereof, it is necessary to form an electric field in the lateral direction. Accordingly, the IPS-type liquid crystal display device is configured to form electrodes on the main surface of the TFT substrate in a comb-teeth shape. No electrodes are formed on the CF substrate and hence, a ratio that an area where the electrodes are not formed within the display region of the liquid crystal panel is increased. Accordingly, particularly, in this type of liquid crystal panel, a large quantity of static electricity is charged to a surface of the polarizer on a CF substrate side and hence, a phenomenon that a display image on the liquid crystal display device is disturbed attributed to the static electricity is liable to easily occur.
FIG. 14 is a cross-sectional view of an essential part for explaining a first example of the conventional structure of the liquid crystal display device for preventing the charging of CF substrate. In FIG. 14, a liquid crystal panel LCD is constituted by adhering a TFT substrate SUB1 and a CF substrate SUB2. To an outer surface of the TFT substrate SUB1 and to an outer surface of the CF substrate SUB2, a lower polarizer POL1 and an upper polarizer POL2 are adhered respectively. A light transmitting conductive layer ITO which is preferably made of indium-tin-oxide is formed between the CF substrate SUB2 and the upper polarizer POL2.
The liquid crystal panel LCD is housed in a resin-made mold frame MLD which holds a backlight not shown in the drawing. The liquid crystal display device interposes a conductive rubber cushion CRC between the liquid crystal panel and an upper frame FL1. The upper frame FL1 is made of metal, and the upper frame FL1 and the conductive rubber cushion CRC are adhered and fixed to each other using a conductive adhesive material AD. A rubber cushion RC is interposed between the TFT substrate SUB1 and the mold frame MLD.
In this constitutional example, by interposing the conductive rubber cushion CRC and the conductive adhesive material AD between a portion of the light transmitting conductive layer ITO formed on the whole surface of the image display screen of the CF substrate SUB2 which is not covered with the upper polarizer POL2 and an inner periphery of the upper frame FL1, and by bringing the conductive rubber cushion CRC and the conductive adhesive material AD into contact with the light transmitting conductive layer ITO and the upper frame FL1 respectively, the light transmitting conductive layer ITO and the upper frame FL1 are electrically connected to each other thus discharging static electricity which is charged to the surface of the CF substrate SUB2 to a ground through the upper frame FL1.
FIG. 15 is a cross-sectional view of an essential part of the liquid crystal display device for explaining a second example of the conventional structure for preventing the CF substrate from being charged with the static electricity. In the same manner as the example 1, the liquid crystal panel LCD is constituted of a light transmitting TFT substrate SUB1 which forms a plurality of drive-use thin film transistors and the like not shown in the drawing on a main surface thereof, a light transmitting CF substrate SUB2 which forms color filters and the like on a main surface thereof, and a liquid crystal sealed in a gap formed between the light transmitting TFT substrate SUB1 and the light transmitting CF substrate SUB2.
A light transmitting conductive layer ITO is formed on the substantially whole surface of the CF substrate SUB2. To most of a light transmitting conductive layer ITO except for a peripheral portion of the light transmitting conductive layer ITO, an upper polarizer POL2 is adhered. In the same manner as the upper polarizer POL2, to an outer surface of a TFT substrate SUB1, a lower polarizer POL1 is adhered. A liquid crystal panel LCD is fixed to a resin-made mold frame MLD which holds a backlight using a rubber cushion RC. A conductive rubber cushion CRC is adhered to the light transmitting conductive layer ITO of the CF substrate SUB2 and the mold frame MLD using a conductive adhesive material AD. Due to such a constitution, the light transmitting conductive layer ITO of the CF substrate SUB2 is electrically connected to the upper frame FL1 made of metal.
The conductive rubber cushion CRC is formed of an approximately strip-shaped elastic body produced by mixing fine particles of a single substance of either one of a conductive material such as carbon or a metal material or fine particles of an alloy of these materials into a silicone rubber material, for example. As an example of another conductive rubber cushion CRC, there has been known a conductive rubber cushion disclosed in JP-A-2004-93718 (patent document 2). A light transmitting conductive layer ITO having a thickness of several hundreds nm is formed on the whole surface of a CF substrate SUB2 by applying alloy consisting of IN2O3 and SnO2 on the whole surface of the CF substrate SUB2 by sputtering. A conductive adhesive material AD is adhered to a surface of the conductive rubber CRC which is brought into contact with the surface of the light transmitting conductive layer ITO and a surface of the conductive rubber CRC which is brought into contact with a part of a stepped portion of the mold frame MLD. Further, an upper polarizer POL2 is arranged on the light transmitting conductive layer ITO. The upper polarizer POL2 is constituted of the multi-layered structure in which a polarization layer, a reflection preventing layer and the like are stacked to each other.
FIG. 16 is a cross-sectional view of an essential part for explaining a third example of the conventional structure of the liquid crystal display device for preventing charging of the CF substrate. Parts indicated by the same symbols used in FIG. 14 and FIG. 15 correspond to identical functional parts. In FIG. 16, a conductive rubber cushion CRC is formed into an inverse U-shape in cross section having an opening portion such as a glass-protecting packing at one end thereof, and an inner surface of the opening portion is mirror-finished. Accordingly, the conductive rubber cushion CRC is hermetically adhered to the respective outer surfaces of the TFT substrate SUB1 and the CF substrate SUB2 of the liquid crystal panel LCD by vacuum suction.
In the opening portion of the conductive rubber cushion CRC, an end portion of a corner portion of the liquid crystal panel LCD is clamped by an elastic force and a vacuum suction force, while another end of the conductive rubber cushion CRC is held and fixed by an elastic force generated between a portion of the mold frame MLD and an inner surface of the upper frame FL1. Due to such a constitution, the light transmitting conductive layer ITO and the upper frame FL1 are electrically connected with each other by way of the conductive rubber cushion CRC outside the display region of the liquid crystal panel.
As documents which disclose the above-mentioned related art, JP-A-2005-77590 (patent document 1) and JP-A-2004-93718 (patent document 2) can be named.
In the above-mentioned related art, it is necessary to ensure a place where the conduction between the conductive rubber cushion and the upper frame is acquired within a predetermined space. Accordingly, it is difficult to apply the related art to a liquid crystal display device of a specification in which a picture frame of a display screen is narrow or a resin molded product having high degree of freedom in shape is not arranged below an upper frame. Further, it is difficult to apply the above-mentioned related art to a liquid crystal display device in which an upper frame is not made of a conductive material or a liquid crystal display device in which black paint or black plating is applied to an upper frame for improving appearance thus lowering conductivity of a surface of the upper frame.